The lunar calendar is repeated every few years and every two months.

The lunar calendar repeats every two months and appears every four years. The second month of the lunar calendar is called leap month, because leap month only happens once every four years, and it is also to balance the dates of the lunar calendar and the solar calendar, so that the dates of the lunar calendar and the solar calendar cannot be too far apart. Originally, the date of the solar calendar was generally about one month different from that of the lunar calendar, and it can only be balanced after the date of the leap month lunar calendar and the solar calendar.

Lunar calendar is a traditional calendar in China. The traditional lunar calendar is based on the lunar calendar and combined with the solar calendar, that is, the combined calendar of yin and yang. The solar calendar is based on the tropical year when the earth revolves around the sun.

Because the pure lunar calendar does not consider the movement of the earth around the sun, the changes of the four seasons have no fixed time on the lunar calendar and cannot reflect the seasons. Unlike the fixed solar year of 365 days or 366 days, the solar year sometimes differs from the lunar year by one month. In order to coordinate the number of days between the lunar year and the solar year, the lunar calendar adopts the "leap method" to adapt to the total number of days in these two years.

Leap month calculation method

Which month of the lunar leap depends on the 24 solar terms in a year. In the China lunar calendar, 24 solar terms are divided into 12 solar terms and 12 neutral terms. At present, the method of setting leap day is between two winter solstices. If it is only 12 months, it is not a leap; if it is 13 months, it is a leap. The leap month begins on the "winter solstice". When there is no "neutral spirit" in the first month, this month is a leap month, and its name is to add the word "leap" before this month.

Generally speaking, every two years or so (every 30 lunar months on average) there will be one month without gas, which coincides with the year when leap months are needed. Therefore, the lunar calendar stipulates that non-neutral months are leap months.